Exercise stimulates angiogenesis in skeletal muscle. The increased capillarity improves muscle function and quality of life in patients with peripheral arterial insufficiency (PAI). Thus, either exercise training or proangiogenic drug therapies (if exercise is contraindicated) could be helpful to these patients. However, the growth factors, receptors, and signaling pathways involved in skeletal muscle angiogenesis are not well characterized. Therefore, these studies will address three Specific Aims: 1) to characterize the time course of alterations in the content and expression of angiogenic growth factors in red and white gastrocnemius muscle of both normal rats and rats with hindlimb ischemia during an exercise training program; 2) to characterize the time course of alterations in the content and expression of receptors for angiogenic growth factors in red and white gastrocnemius muscle of both normal rats and rats with hindlimb ischemia during an exercise training program; and 3) to determine whether a functioning nitric oxide signaling pathway or the angiopoietin/Tie-2 system is essential for exercise- stimulated angiogenesis. These studies will utilize femorally- ligated rats, a model of PAI. Rats will follow a treadmill exercise program known to stimulate angiogenesis. For Specific Aims 1 and 2, levels of growth factors (VEGF, bFGF, angiopoietin- 1, and angiopoietin-2) and their receptors will be assessed over the initial period of vascular expansion (28 d) and correlated with capillary growth. For Specific Aim 3, the effect of nitric oxide synthase or Tie-2 (angiopoietin receptor) inhibition on capillary growth and cell proliferation will be evaluated following approximately 2 w of exercise. These studies will advance understanding of skeletal muscle angiogenesis and may suggest treatments for patients with PAI.